Multicarrier modulations associated with error correction coding and interlacing have demonstrated their advantages particularly for high speed transmissions in a radio mobile environment as illustrated in patents FR-86 09622 and FR-95 05455 dealing with the OFDM (Orthogonal Frequency Division Multiplexing) modulation and IOTA (Isotropic Orthogonal Transform Algorithm) modulation.
In the case of a coherent demodulation of received data, it is essential to be able to make a good estimate of the channel at all points in a time-frequency plane, in order to correctly demodulate the information received by the receiver.
In particular, the invention relates to optimisation of channel estimating techniques in the case of a multiple carrier transmission, called multicarrier transmission.
Usually, when data are transmitted on a radio channel at high speed, the signal is subjected to Doppler effects (related to movements of the transmitter, receiver or any reflectors) and multiple path problems (for example related to reflection of the signal on different objects) causing a delay spread or variations of the signal amplitude. These effects depend particularly on the time and frequency considered. Thus, a channel with a variable time and frequency is complex and it has to be estimated correctly in order to be able to decode received data reliably.
According to the state of the art, in the case of a multicarrier modulation, the channel estimate from which degradations caused by a radio mobile channel can best be corrected, consists of the following steps:                insert reference carriers into the flow of information carriers at locations known to the receiver;        making use of values taken by these references in reception, to deduce the channel transfer function at these points in the time-frequency plane; and        starting from these results, obtain the channel transfer function on all points in a time-frequency network.        
There are two particular methods of doing this:                the estimate by scattered pilots; in transmission, pilot symbols are distributed regularly in the time-frequency plane. An undersampled version of the channel is obtained starting from these pilots. In reception, a two-dimensional interpolation is carried out in time and frequency to determine the value of the channel at all points in the time-frequency network. This method is used particularly by the DVB-T (Digital Video broadcasting —Terrestrial) standard.        the estimate by reference multicarrier symbol (also called preamble); at least one reference symbol is placed at the beginning of a frame emitted on the channel. With this or these symbols, the channel is estimated in reception of the frame transmitted on each carrier. The channel can be considered as being quasi-static on a given frame (choice of system parameters such that the channel varies slowly compared with the symbol time), the channel estimate on the reference symbol(s) is valid for all OFDM symbols in the frame (in particular this method is applied to the ETSI (European Telecommunication Standard Institute) HIPERLAN/2 standard.        
In the case of a scattered pilots estimate, the complexity of the channel estimator is usually limited by using two single-dimensional interpolation filters instead of a two-dimensional filter. The implementation complexity is then significantly lower for an acceptable degradation of the estimating quality.
For example for DVB-T, a Wiener filter is used for the frequency interpolation while the time interpolation is a simple linear interpolation. The linear interpolation is not optimum but its complexity is lower than the complexity of a two-dimensional FIR filter or two single-dimensional FIR filters in cascade.
Therefore, the interpolation is done in time and then in frequency.
Furthermore, it is difficult to make a Wiener filter adaptive, in other words to recalculate the coefficients of the reception filter as a function of the channel parameters. Consequently, it systematically becomes necessary to size the filter for the worst Doppler and delay spread case.
In particular, the purpose of the invention is to overcome these disadvantages of prior art.
More precisely, one purpose of the invention is to optimise decoding of a data frame emitted on a channel with a strong Doppler spread when multicarrier modulation is used.
Another objective is to optimise the estimate of the transfer function of a transmission channel within the framework of a multicarrier transmission.
Another purpose of the invention is to enable a fairly simple interpolation of the channel transfer function over an entire time-frequency network starting from estimated values of this function at locations of pilot symbols inserted in the transmitted data frame.
Another purpose of the invention is to offer a good compromise between the useful flow and the quality of the channel estimate.